Title: What’s
the Difference between Energy and Power?
Author: Frederick A. Ringwald (Department of Physics, California State University, Fresno)
(E-mail: [email protected]).
Abstract: This module will show the difference between energy and power. It will do this with
examples from the Sun and in the deep Universe, and also with examples from everyday life.
Audience: Undergraduate general-education, introductory astronomy for non-majors,
undergraduate general science, and K12 science. A pre-algebra mathematical background is
assumed, involving multiplication and units. A Flesch-Kincaid readability test shows that the
module is written at a grade level of 9.0.
Educational Goals: This module will show the difference between a solar prominence and a
solar flare. It will then use the prominence and the flare to explain the difference between
energy and power. This will illustrate a central principle in science: that physical law is universal,
with the same laws valid everywhere in the Universe.
Module Type: This module is designed to be flexible. It can be used either as an in-class or lab
activity, a worksheet, or as a homework assignment.
Module Outline: See the abstract above and the module itself, since the module is only four
pages long.
Resources Needed: A basic, four-function calculator would be useful.
References: Energy: Its Use and the Environment, 5th ed., by Roger A. Hinrichs and Merlin H.
Kleinbach (Cengage Learning, 2012); also Astronomy for Beginners, preliminary editon, by
Frederick A. Ringwald (Kendall-Hunt, 2013) or any other fine introductory astronomy text.
What’s the Difference between Energy and Power?
Objectives: This module will show the difference between energy and power. It will do this
with examples from the Sun and in the deep Universe, and also with examples from everyday
life. This will illustrate a central principle in science: that physical law is universal, with the
same laws valid everywhere in the Universe.
Figure 1: A solar prominence is hot gas rising from the Sun (NASA/ESA/SOHO).
Figure 1 shows a solar prominence. A prominence is a streamer of hot gas rising from the Sun.
They can look like loops, or like flames. Prominences can extend a million miles into space:
notice how Figure 1 shows the size of Earth, to show just how big a prominence can be. Of
course, Earth is never this close to the Sun: this is just a size comparison. A prominence can last
for about 8 hours. It’s fun to use a safe solar telescope to watch prominences come and go, over
an afternoon.
Figure 2: A solar flare is a violent explosion on the Sun (NASA/SDO).
Figure 2 shows a solar flare. Flares are violent, often having 10 times more energy than solar
prominences. Solar flares also last only about 5 minutes. This means that solar flares are
explosive, releasing their energy in 10 times less time than solar prominences do.
Solar flares are therefore 100 times more powerful than solar prominences. This is because solar
flares have 10 times more energy, and release it in 10 times less time. In other words:
Power = energy/time.
In other words, power is how fast you use energy.
Why should anyone care about this? One reason is that it’s practically useful. Remember that
physical law is universal: the Universe follows orderly laws that we observe to work the same,
throughout the Universe.
Name: ______________________________
Day: ________________________________
Time: ______________________________
Lab Instructor: _______________________
Worksheet: What’s the Difference between Energy and Power?
Using what was written on the previous two pages, you can count up all the electrical energy that
you use in a day. To do this, you need to know how much power your electrical devices use, and
how much time you use them in a typical day.
Think about your dorm room, apartment, home, or whatever residence you live in, and fill in
your estimates of how many hours per day, on average, you use the following electrical devices:
Power
Average
× time in use
=
per day
× _______ hours =
× _______ hours =
× _______ hours =
× _______ hours =
× _______ hours =
× _______ hours =
× _______ hours =
× _______ hours =
× _______ hours =
× _______ hours =
× _______ hours =
× _______ hours =
Refrigerator
75 Watts
Electric alarm clock
10 Watts
Hair dryer
500 Watts
Reading lamp
60 Watts
Ceiling light
100 Watts
Desktop computer
100 Watts
Printer
50 Watts
Air conditioner
200 Watts
Space heater
600 Watts
Toaster
100 Watts
Microwave oven
100 Watts
Charger for a cell phone
5 Watts
(or mobile phone)
Charger for a digital music player
7 Watts × _______ hours =
(such as an iPod)
Charger for a tablet computer
10 Watts × _______ hours =
(such as an iPad)
Television
100 Watts × _______ hours =
Video game controller
2 Watts × _______ hours =
Charger for a laptop computer
65 Watts × _______ hours =
Stereo or other loud music player
100 Watts × _______ hours =
Washing machine
100 Watts × _______ hours =
CONTINUED ON NEXT PAGE
Energy used
per day
(Watt-hours)
___________
___________
___________
___________
___________
___________
___________
___________
___________
___________
___________
___________
___________
___________
___________
___________
___________
___________
___________
CONTINUED FROM PREVIOUS PAGE
Clothes dryer
300 Watts × _______ hours =
Other 1 ______________________ ___ Watts × _______ hours =
Other 2 ______________________ ___ Watts × _______ hours =
___________
___________
___________
SUM of electrical energy used per day, in Watt-hours =
___________
This is useful and practical, since the electric company calculates your electric bill in kilo-Watthours, abbreviated kWh, where 1000 Watt-hours = 1 kWh.
(For a more extensive list, see Energy: Its Use and the Environment, 5th ed., by Roger A.
Hinrichs and Merlin H. Kleinbach, 2012.)
Hint 1: Most refrigerators are on for 24 hours per day. Many desktop computers are left on for
24 hours per day. Hair dryers are used for only about 10 minutes per day, so while they use lots
of power (in other words, they use energy very fast), they don’t use as much energy as other
appliances use, since they’re not used all day.
Hint 2: Remember that: 5 minutes = 1/12 hour = 0.083 hours,
10 minutes = 1/6 hour = 0.167 hours,
15 minutes = 1/4 hour = 0.25 hours.
Hint 3: An average American household uses about 20 kWh per day.
QUESTION: Which of the electrical devices listed above uses the most power? ____________________
QUESTION: Which electrical device listed above uses the most energy, in a day? __________________
QUESTION: Which of the electrical devices listed above uses the least power? ____________________
QUESTION: Which electrical device listed above uses the least energy, in a day? __________________
Another example of the difference between energy and power is the Saturn V moon rocket. The
first stage of the Saturn V was the most powerful machine ever built, 85 times more powerful
than Hoover Dam, near Las Vegas. The Saturn V’s first stage only fired for 12 minutes, though,
in order to fly into orbit. Hoover Dam may be 85 times less powerful, but it has been in the
Colorado River near Lake Mead since 1936. Hoover Dam has generated much more energy,
since although it’s less powerful, it has operated for a much longer time.
One more example of the difference between energy and power are quasars and gamma-ray
bursts. Quasars are the most energetic things known in the Universe: they shine with the power
of 10,000 galaxies for millions of years. Gamma-ray bursts are the most powerful explosions
known: one gamma-ray burst was more powerful than the whole rest of the Universe, but for less
than two seconds. This gamma-ray burst didn’t have as much energy as a quasar, even though it
was much more powerful—but only for two seconds.